Sediment Storage in the Colorado River Downstream from Glen Canyon Dam

Introduction

The sandbars exposed along the shoreline of the Colorado River represent only a small fraction of the sand deposits in Grand Canyon, most of which are on the bed of the river in eddies and the channel. Current management practice includes efforts to maintain and build sandbars by releasing high flows from Glen Canyon Dam that are timed to coincide with periods of fine-sediment supply from tributaries (High-flow Protocol Environmental Assessment). The success of this approach to build sandbars depends on the maintenance of a sufficient supply of sand within the channel. The purpose of the sediment-storage monitoring project is to track long-term trends in sand storage and thereby provide a robust measure of whether or not the supply of sand available for building sandbars is increasing, decreasing, or remaining stable over time-scales of years to decades.

Monitoring Sediment Storage

We measure changes in sediment storage by making repeat topographic maps of the river bed and banks. The maps are made by surveying exposed sediment deposits with conventional total station. These measurements involve the use of a survey instrument set on a known elevation (also called a benchmark) to measure the location and elevation of points on the ground selected by a rodman equipped with a reflective target. Most of the sediment is underwater and is measured with sonar. Multibeam sonar is the most efficient method to measure this sediment, because it is capable of mapping wide swaths of the riverbed. Singlebeam sonar measures depths directly below the instrument and is used to map areas too shallow for the multibeam equipment, but too deep for conventional survey. All of the sonar measurements are positioned by shore-based robotic total stations that track boat position in real time. GPS is not used for any of the measurements, because satellite signals are not sufficiently reliable in the deep canyon environment.
Complementary measurements of changes in sediment storage are also made by measuring sediment concentration in the water (Discharge, Sediment, and Water Quality Monitoring).

Surveying an exposed sandbar along the bank of the Colorado River in May 2014.

Boat equipped with multibeam sonar surveying the bed of the Colorado River in May 2009. Instrument in foreground is robotic total station that automatically tracks boat location and radios the position to a computer on the boat 20 times per second. Boat equipped with singlebeam sonar for mapping the bed of the river at shallow depths along the shoreline of the Colorado River.

Recent Findings

Initial results indicate that sand storage did not decline between 2002 and 2009. This period was one of average to above average tributary sand inputs and average to below average release of water from Glen Canyon Dam. These findings are based on a period that was favorable to sand accumulation. Periods when dam release volumes are greater and tributary sediment inputs are less frequent will likely result in less sand accumulation. Recent results also demonstrate that measurements of channel change made in short reaches (less than a few miles in length) can be used to track changes in deposits and transfers of sand among the storage locations within the short reaches. The results, however, cannot be extrapolated to long segments of the river (over 10 miles in length), because the size and distribution of sand storage locations is highly variable. These findings are explained in detail in an article in the Journal of Geophysical Research and summarized in a U.S. Geological Survey Fact Sheet.

Perspective view of digital elevation model of the bed and banks of the Colorado River about 44 miles downstream from Lees Ferry, Arizona. Between 2009 and 2012, it is possible to see erosion of sediment from the bed of the river in the channel and erosion of sediment from the sandbar on the bank. The direction of streamflow is from the upper left to lower right and the river is about 450 feet wide at the widest point in this view.

Project Chief

The data presented in this website are collected
and processed using standard USGS protocols
and other established peer-reviewed methods,
and subject to rigorous quality control.
Nevertheless, minor edits of these data are
possible.

The data are released on the condition that neither the USGS
nor the U.S. Government may be held liable for any damages
resulting from its authorized or unauthorized use.